Audio amplifier has wide-spread application in all electronic products that need audio output, for example, walkman, audio set, MP3 player, PDA and cellular phone, etc. The conventional Audio amplifier can be categorized as A type, B type, AB type and D type, etc. Among them, A type, B type and AB type amplifier are the so-called linear amplifier and class D amplifier is nonlinear amplifier called pulse width modulation (PWM) amplifier.
A type amplifier usually just has one active component, for example, transistor. The transistor needs a bias circuit, therefore, no matter how large the input signal is, it can not be in fully conducted or fully non-conducted state and this conducted/non-conducted region is the so-called linear region. A type amplifier has the advantage of high linearity in its response and thus low loss of fidelity in its output, which makes it suitable for high fidelity audio system. However, A type amplifier has very low efficiency with a theoretical power efficiency of about 25% but an actual power efficiency of only about 15% to 20%. Additionally, A type amplifier will generate large amount of heat during the use at large power, it is thus not suitable to be used as portable equipment.
B type amplifier is generally composed of two transistors pushing each other, one outputs the current and the other absorbs the current and the effect of amplification is achieved through alternate conduction of these two transistors. B type amplifier has larger power efficiency than that of A type amplifier, that is, it has a theoretical power efficiency of about 78% and the real power efficiency is in between 50% to 70%. However, when the input signal is smaller than the conduction voltage of the transistor, both transistors will be at non-conducted states, this is the so-called intermodulation distortion. Therefore, B type amplifier is not linear in the whole dynamic range.
Additionally, AB type amplifier is a combination of A type and B type amplifier. Its structure is very similar to B type amplifier but a circuit which can provide small bias current to each transistor is adopted; therefore, each transistor will not be in fully non-conducted state. Its power consumption is larger but the intermodulation distortion can be almost eliminated. Its operation is similar to B type amplifier and two transistors are associated to complete the mission, however, the overall performance is a little bit better than that of B type amplifier. The theoretical power efficiency of AB type amplifier is about 78% but the actual value is in between 50% to 70%.
The common disadvantages of the above mentioned A type, B type and AB type amplifier are the needs of good heat-dissipating equipment and space. Therefore, they are mostly bulky and the operation consumes a lot of power. Under the trend of low power consumption, small form factor and light weight pursued by most electronic products, those linear amplifiers thus are not suitable to be used in such electronic products.
Therefore, CLASS D AMPLIFIER is thus developed with an efficiency higher than the above mentioned linear amplifier (class D amplifier has a theoretical power efficiency of about 100% but the actual power efficiency is still higher than 85%) to meet the above mentioned needs. Class D amplifier can save more power and improve the lifetime of battery due to its high power efficiency. Moreover, class D amplifier generates lower heat during its operation and thus can reduce the energy control cost. What's even more is some class D amplifiers do not need heat-dissipating devices and thus the volume of equipment which adopts that kind of amplifier can be much smaller.
Currently, lots of literature discloses several improved class D amplifiers in an attempt to further increase the power efficiency and enhance the stability of class D amplifier; moreover, its circuit is further simplified and its volume is even smaller and the fidelity loss is even fewer. For example, U.S. Pat. No. 4,689,819 discloses how to make the CMOS device of class D amplifier more efficient under smaller volume and compatible battery set; U.S. Pat. No. 5,317,640 discloses how class D amplifier can reduce the current and reduce in turn the power consumption when the input signal is zero, this makes it suitable to be used in the driving of mono channel or hearing aids; U.S. Pat. No. 6,016,075 discloses a class D amplifier which can reduce cost and DC component error; U.S. Pat. No. 6,924,700 discloses a class D amplifier with correction circuit; U.S. Pat. No. 7,078,964 discloses class D amplifier having DC current detection circuit.
However, in the prior art, when class D amplifier is used to drive dual channel tri-wired stereo amplifier, half-bridge circuit architecture needs to be used to realize this idea; since this method needs the supply of bipolar power for its operation, it is thus not suitable to be used in portable products. If it is to be used in portable product and uni-polar power, such as battery, is used as the power supply, then a generator circuit of half voltage point is needed to drive the amplifier; since the half voltage point generator circuit is analog circuit, temperature drift, input voltage drift and process drift must be considered during the designing stage, the output voltage is thus easily changed by different working environments, which might further lead to power consumption due to the need of extra circuit and might affect the audio output quality of the entire system.